I find it hard to write a review without writing anything about the content. I don't think I point out any big secrets below, but if you want to read the book without any spoilers, it might be best to skip the rest of the review. If that is the case here a short form: I liked the book and recommend reading it.
Obviously this genre is romance, but still: One thing that annoys me with many of these novels is the focus that is placed on the romance storyline, sometimes to the exclusion of all else. Some stories have side-stories, but they usually flow into the main stream of the story at some point and one gets the feeling that in that world nothing happens that does not impact the main story in some way.
In "It Happened One Wedding" both protagonists have a life outside the main story and we see parts of it, even if it has no bearing on the romance. This allows the reader to get a feeling of who the characters are and makes the story more realistic.
The romance itself is sweet and believable. I dislike novels where the emotions seem forced or the story is unbelievable. That is not the case here. I liked both characters and wanted them to be together. They also had nice scenes with sassy and funny dialogue.
I am not a fan of big misunderstandings or the multitude of protagonists in romance novels who all seem to have PTSD. As the description of this novel already points out, Sidney suffered a "humiliating end to her engagement", but by the time the novel starts she already has adapted to some degree, while there isn't really anything major for Vaughn. There are no big misunderstandings, another plus.
That probably leads to one of the not so stellar points of the book: Vaughn is portrayed as the bachelor type, but there is no really convincing reason as to why he would not want to commit. It took a bit too long for him to realise he was in love with Sidney - if he had reason for not wanting to admit his emotions to himself I might have understood, but there wasn't really anything in his back-story that would make him shy away from commitment or love. If he was this determined bachelor, the one-year relationship he had in the past would make no sense - apparently he has no problem with a longer relationship. So I didn't really get what took him so long to realize/admit his feelings and act on them.
Another not so stellar point is that the side stories did not get wrapped up. I guess it could imply that their lives / the side-stories go on (and have no ending as such), but in the case of Vaughn the side-story was an FBI investigation close to completion by the end of the book and it would have been nice to know how it ended.
In terms of grammar and vocabulary the book was good. Overall I enjoyed the story and would recommend reading the book.